We have been studying the effects of various brain lesions on the performance of a visual attention task. Normal subjects and patients fixated on a spot of light and responded to peripheral target lights. Reaction times in this task were fastest when the target was preceded by a cue at its location (valid cue); times were slower for cues at other locations (invalid cues). On some trials the whole visual field was weakly illuminated (diffuse cue). Humans with lesions of parietal cortex performed well on most aspects of this task but were severely slowed when the cue drew their attention into their intact visual field and they had to respond to the target in their affected field. They also responded slowly to targets in both fields after diffuse cues. These data suggest that patients who have damage to their parietal cortex have a specific difficulty in shifting their attention once their intact visual field has captured their attention. No other patient population showed this effect. Humans with damage to the frontal lobes were only slowed when a diffuse cue was presented prior to a target in their affected visual field. People diagnosed as schizophrenics had slowed responses after diffuse cues prior to targets in their right visual field. Persons with occipital lesions which caused a hemianopsia showed no influence of the cue when it was presented in their "blind" visual field; this suggests that the attentional system is dependent on data arising from occipital cortex. These studies are important in finding the specific contribution of brain areas to visual attention, and this approach may eventually be of diagnostic value for certain brain lesions.